error[E0369]: cannot multiply `Thing` by `{integer}`
  --> $DIR/issue-3820.rs:14:15
   |
LL |     let w = u * 3;
   |             - ^ - {integer}
   |             |
   |             Thing
   |
note: an implementation of `Mul<_>` might be missing for `Thing`
  --> $DIR/issue-3820.rs:1:1
   |
LL | struct Thing {
   | ^^^^^^^^^^^^ must implement `Mul<_>`
note: the following trait must be implemented
  --> $SRC_DIR/core/src/ops/arith.rs:LL:COL
   |
LL | / pub trait Mul<Rhs = Self> {
LL | |     /// The resulting type after applying the `*` operator.
LL | |     #[stable(feature = "rust1", since = "1.0.0")]
LL | |     type Output;
...  |
LL | |     fn mul(self, rhs: Rhs) -> Self::Output;
LL | | }
   | |_^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0369`.